This invention relates to methods for testing electronic circuits and more particularly to methods for quickly and efficiently testing integrated circuit devices having high output voltages.
With the great multitude of new electronic circuits, and particularly integrated circuit devices, and the vast quantities being produced, it has become imperative that efficient means for testing such circuitry be utilized to ensure that only good circuits are sold to customers. Toward this end, a number of manufacturers have been producing and selling computerized electronic test equipment, such as the well-known Sentry.RTM. unit, manufactured and sold by Fairchild Camera and Instrument Corporation. Such electronic test equipment can be programmed to provide a variety of electronic signals to a device under test (DUT) and measure a variety of electrical signals from that device under test, and utilizing this information, determine whether the device under test meets the manufacturer's and customer's specifications. For example, the Sentry tester is capable of providing supply voltages and appropriate address and enable signals (if required to the device under test), and measuring the output signals from the device under test. More particularly, it is important to test the logical 0 ("low") and the logical 1 ("high") output voltage levels of the device under test. It is also important to test these logical 1 and logical 0 voltage levels at maximum load current. In other words, with a low output signal, the device under test must be able to sink a specified amount of current and simultaneously provide a low voltage output signal less than a specified voltage VOL. Conversely, the device under test must be able to source a specified amount of current and simultaneously provide a high voltage output signal greater than a minimum voltage VOH. In many integrated circuits, the logical 1 output signal is a relatively low voltage, typically 5 volts for TTL devices, or slightly greater for other devices. Most computerized test equipment is designed to measure low voltages quite handily; however, these test systems are designed to measure voltages only up to approximately 20 volts, since that is greater than the voltages most commonly encountered in integrated circuit applications.
A new breed of semiconductor devices is currently being manufactured, having significantly greater output voltages than 20 volts. For example, the S4534, and S4535, manufactured by American Microsystems, Inc., have an output voltage of as great as 60 volts. These devices are typically used in such special applications as driving fluorescent displays, and thus are becoming quite popular. However, conventional computerized test equipment does not provide means for accurately measuring such high output voltages. In the past a simple method for measuring output voltage has been to halt the operation of the computer program used to test the device at specific steps in the test program in order to allow an individual to measure the output voltage, by such means as using an oscilloscope or a voltmeter. However, this technique is unfeasible for large scale production where testing might be done quickly, without highly skilled labor, and with good quality.
Another prior art technique for measuring high output voltages is the use of a resistor divider network to divide the high output voltage down to a voltage within the range capable of being accurately measured by computer test equipment. However, the use of a resistor divider has several disadvantages. First, the resolution of the computer test equipment in conjunction with the resistor divider is significantly less than the resolution of the computer test equipment alone. For example, if the resistor divider divides the high output voltage by a factor of 10, the resolution of the computer test equipment in conjunction with the resistor divider is only 1/10th the resolution of the computer test equipment alone. If the resolution of the computer test equipment is .+-.1 millivolt, then the resolution of the computer test equipment in determining the voltage levels of the device under test is only .+-.10 millivolts. Another disadvantage of using a resistor divider network is that the resistances of the divider must be highly precise in order to provide a precise division of the high voltage to be measured. Such high precision resistors are very expensive, and are unlikely to yield exactly the desired resistance ratio. Furthermore, if a number of test boards are to be manufactured, it is highly unlikely that each board will exhibit the same division factor, thus requiring recalibration of the computer test system whenever a test board is changed. Furthermore, a single test board may test a plurality of output signals from a device under test, requiring the use of a plurality of resistor dividers, each with their own unique division factor.